Determining the proper size bike for a child is based on the size of the bike's wheels as opposed to determining size for adults, which is typically measured using the frame. Unless a child is unusually tall or short for his or her age, there are standards for every age beginning from the age of 2. Once the basic size is determined, then other amenities can be selected, such as number of speeds, types and shape of the handle bars, types of seats and how the brakes work.
For children between the ages of 2 and 4, the typical wheel size is 12 inches, which is ideal for a child with an inseam of between 14 and 17 inches. These bikes typically have training wheels and work with direct drive.
For children from ages 4 and 6, the wheel size increases to 14 inches, which works best for children with inseams between 16 and 20 inches. For children between the ages of 5 and 9 years, a 16-inch wheel is common. These bikes work for children with inseams between 18 and 24 inches and often have pneumatic tires and rear coaster brakes. Children with inseams between 22 and 25 typically ride bikes with 20-inch wheels, which works best for children between 7- and 10-years-old. These bikes begin typically have multiple speeds and commonly come with hand brakes.
Children age 9 and older, with inseams between 24 and 28 inches can ride bikes with 24-inch wheels. Once children begin to reach their teenage years, they can begin to ride bikes geared for adults, based on height.